CD4 T cells have long been recognized as crucial component of the protective immune response to M. tuberculosis. HIV+ subjects have a greatly increased susceptibility to tuberculosis, and mice deficient in CD4 T cells are impaired in their control of M. tuberculosis infection. It is generally believed that the major effector mechanism of CD4 T cells in tuberculosis is the production of IFN-gamma for activation of macrophages and subsequent killing of intracellular organisms. Although IFN-gamma production is undoubtedly essential to control of this infection, CD4 T cells likely have other roles in the protective response to M. tuberculosis. In a murine model of chronic tuberculosis, depletion of CD4 T cells resulted in fatal reactivation of the infection. However, overall levels of IFN- gamma in the lung were unchanged, and inducible nitric oxide synthase (NOS2) expression and activity were similar to control mice. Thus, the chronic M. tuberculosis infection was not contained in the absence of CD4 T cells, even in the face of wild type levels of IFN-gamma and reactive nitrogen intermediates. These data suggested the presence of CD4 T cell-mediated effector mechanisms in addition to IFN-gamma and activation of RNI production by macrophages. This proposal focuses on identifying the roles that CD4 T cells play in host defense against M. tuberculosis, apart from IFN-gamma production. Aim 1 focuses on a role for CD4 T cells in maintaining a functioning CD8 T cell response in the lungs. Aim 2 is an investigation of the role of CD4 T cells in macrophage activation, focusing on known potential components, including the relative importance of IFNgamma, CD40, and NOS2. Aim 3 focuses on the spatial relationship of T cells and macrophages within the granuloma, and how the loss of CD4 T cells affects this cell-cell interaction. Finally, Aim 4 is a broad approach to identifying previously unknown CD4 T cell- mediated antimycobacterial mechanisms in in vitro and in vivo models, using gene expression technology. This collaborative project takes advantage of the availability of mouse strains and reagents, as well as the Principal Investigators' previous collaborative work and extensive experience in acute and chronic murine models of tuberculosis. The results obtained are relevant to vaccine design and efficacy, as well as to understanding treatment and prevention of tuberculosis in HIV+ (and therefore CD4 T cell deficient) individuals.